I'm putting this out there to see if anyone will make one. It is an air-rifle with a barrel that runs concentric to the air-chamber, and has a rearward moving piston. Loading is direct at the breech which is sealed by a falling-block or swing-out breech. Cocking would be by an underlever with linkages down and forward, or sideways and forward in the case of a sidelever version. Calibre would be easily interchangeable. The piston could be sealed by TWO 'O' rings, one for the inside and one for the outside edge.

I know there was a pistol with a concentric barrel, was this the Acvoke?

Anyway, maybe it would not make the top of the sales charts but I bet most collectors would want to have one in their collection!